1. Field of the Invention
The present invention relates to a method of measuring a length and an apparatus therefor, wherein power consumption can be reduced and a secondary output can be multiplied.
2. Description of the Related Art
A dial gauge as one of precision gauges is a typical length measuring apparatus widely used in a machine industrial field. A main body is fixed to a predetermined position with respect to a reference position or a reference surface. A probe extended from the main body is reciprocatedly held and is brought into contact with the reference surface or a surface of an object to be measured. As a result, a position of the probe is displaced and retracted. A retracting amount is transmitted to an indicator of the main body to cause an indicating needle to swing, so that a length can be measured on the order of 1/1,000 mm. In addition, the above apparatus is utilized in measurement of the diameter and circularity of the object to be measured.
In the measuring apparatus as described above, digital apparatuses are recently available. In this apparatus, a rod-like core having a predetermined size is bonded in tandem with the probe so as to coincide their axial directions. An end portion of the core is compressed by a compression spring so that the probe is outwardly biased. The probe including the core is supported so as to be movable in the axial direction. Coils La, L, and Lb are arranged in a moving area of the core in accordance with the displacement of the probe brought into contact with the object to be measured. Coil L is arranged in the center of these coils. Coils La and Lb are respectively arranged at both ends of coil L. The core can be freely moved through coils La, L, and Lb. FIG. 1 shows an equivalent circuit arrangement of this apparatus. Coil L serves as a primary coil, and coils La and Lb each having the identical standards serve as secondary coils. Inversely series-connected secondary coils La and Lb constitute a differential coil arrangement.
An RF current having a high frequency of about 10 kHz is supplied to primary coil L, and outputs from secondary coils La and Lb are obtained as outputs corresponding to a position of the core. Since secondary coils La and Lb are differentially wound, a voltage difference therebetween can be obtained.
Since the core influences inductances of the secondary coils, the voltage difference between secondary coils La and Lb reflects a position of the core. Therefore, secondary output voltage Vout is detected, so that a moving amount of the probe can be measured.
The conventional length measuring apparatus as described above has an arrangement wherein an output difference is obtained as a measuring amount. In addition, since induction voltages of secondary coils La and Lb are about 1 V each, the obtained voltage difference is small. Therefore, an amplifier is necessary. In order to assure high measuring precision, the amplifier having good temperature characteristics is required. However, the amplifier having the good temperature characteristics is expensive, thus increasing cost of the apparatus.
In the length measuring apparatus of this type, a built-in battery is employed as a power supply so that the apparatus can be readily used anywhere as a handy-type apparatus. For this reason, a high impedance of the coil is required. Since the impedance of the coil is in proportion to the number of turns of the coil, the coil must have a large size in order to obtain a high impedance and to reduce power consumption, thereby causing an increase in size of the apparatus.